Organic thin-film solar cells can be produced by non-vacuum processes and hence have a merit of being mass-producible at low cost by simple coating processes such as printing, in contrast with inorganic material-based solar cells such as silicon solar cells and, for example, CIGS type of compound semiconductor solar cells. Further, since being in the form of thin films, they can serve as devices on flexible substrates of resins or the like. In addition, since they are also light in weight, it is possible to utilize their flexibility for designs thereof with high degree of freedom. Because of those advantages, the organic thin-film solar cells are expected to be next-generation solar cells. However, at present, their conversion efficiencies and working lifetimes are inferior to those of conventional solar cells, and accordingly it is said to be necessary to develop new device structures and/or materials.
In an early stage of the development, organic thin-film solar cells produced experimentally were of the pn-heterojunction (plane heterojunction) type comprising p- and n-type organic semiconductors. However, since the exciton diffusion length therein was as short as about ten nanometers, carriers were generated only within an area ranging from the pn-junction interface to some dozen nanometers depth and accordingly they had very low conversion efficiencies.
In order to improve the conversion efficiencies of the above solar cells, bulk heterojunction (BHJ) technology is developed in which p- and n-type of organic semiconductors are blended so that the pn-junction interface of nano-order may be dispersed in the whole thin film. This technology is regarded as a breakthrough giving remarkable improvement of the conversion efficiency. FIG. 1 shows a schematic sectional view of a bulk heterojunction type solar cell, which comprises a substrate 10, an anode 11, a hole transport layer 14, an active layer (photoelectric conversion layer) 13, an electron transport layer 15, and a cathode 12. Further, FIG. 2 shows a conceptual drawing illustrating the working mechanism of the bulk heterojunction type solar cell.
The photoelectric conversion process in an organic thin-film solar cell is said to undergo the following steps:
(1) light absorption and exciton generation by organic molecules,
(2) migration and diffusion of the excitons,
(3) charge separation of the excitons, and
(4) charge transportation to both electrodes.
In the step (1), a p- or n-type organic semiconductor absorbs light to generate excitons 20. The generation efficiency in this step is hereinafter represented by “η1”. Successively in the step (2), the generated excitons migrate to the D/A hetero interface by diffusion. The diffusion efficiency in this step is hereinafter represented by “η2”. Since having lifetimes, the excitons can migrate only in as long a distance as the diffusion length. In the following step (3), the excitons reaching the pn-junction interface are then separated into electrons 21 and holes 22 (i.e., free carriers). The separation efficiency in this step is hereinafter represented by “η3”. Lastly, in the step (4), the free carriers are individually transported to the electrode through the p- or n-type organic semiconductor material and then introduced into the external circuit. The transportation efficiency in this step is hereinafter represented by “η4”.
Consequently, the external extraction efficiency ηIQE of the carriers based on the applied photons can be represented by the formula of: ηIQE=η1·η2·η3·η4. This value corresponds to the quantum efficiency of the solar cell.
Accordingly, in order to increase the photoelectric conversion efficiency of organic thin-film solar cell, it is necessary to improve each of the steps (1) to (4).
Specifically, in the step (1), the active layer is required to absorb incident photons as efficiently as possible, preferably at the rate of 100%.
In the steps (2) and (3), the organic semiconductor materials are required to enhance mobility of the carriers and to ensure the pn-junction.
Further, in the step (4), it is required to form such carrier paths leading to the electrodes that the distance to each electrode may be shortened and it is also required to reduce defects acting as traps. If those requirements are satisfied, the photoelectric conversion efficiency can be improved.
Accordingly, if an organic thin-film solar cell is so produced that the above requirements may be satisfied, a device of high efficiency can be realized. However, as long as the production is based on practically available materials and film-forming methods, the requirements are difficult to satisfy sufficiently. For example, the organic p-type semiconductor polymer now adopted in the active layer has such a narrow absorption spectrum width that it can absorb light only within a particular small wavelength range. This is a large obstacle at present to improve the efficiencies of organic thin-film solar cells.